Converting biomass into fluid hydrocarbon products is an increasingly relevant activity in order to provide sustainable industry, to minimize use of fossil fuels, and to provide energy and chemical feedstock security. Converting biomass into hydrocarbons can generate a stream of fluid products that is more fungible, economic and transportable than the original biomass. The most advantageous manufactured hydrocarbon products are those that are able to meet existing industrial standards for commodities such as natural gas, natural gas liquids, transportation fuels, alkenes and other exportable products.
Biomass harvesting and transportation is often more costly and energy intensive than fossil fuel feedstock collection. Biomass as grown is also a relatively low density source of chemical energy. This drives the need for biomass conversion processes to efficiently conserve carbon and energy.
Thermochemical conversion of biomass to useful fuel or chemical molecules generally involves a thermal decomposition process, such as gasification or pyrolysis that volatilizes carbonaceous material, plus a chemical process which convert the vapours to an end product. The gasification process generally uses an oxidant, such as air or oxygen, to generate heat by combustion of a portion of the feed biomass in order to provide energy for the initial volatilization of the biomass. Pyrolysis, on the other hand, uses an external source of energy to volatilize the biomass, and does not introduce an oxidant to the process stream. Both methods of volatizing biomass yield a mixture of gases, vapours, aerosols, and solids (char). A substantially fluid stream is processed to form target fuels and chemicals, while the solids generated are separated from the target fuels and chemicals. Pyrolysis followed by rapid cooling of the vapours and aerosols results in the formation of bio-oil.
However, thermochemical conversion practitioners have been plagued with small but significant co-production of viscous, condensable compounds which tend to deposit and adhere to downstream equipment and reactors where the fluid reactant streams cool. These compounds are generally called tars. Tar, for this disclosure, means compounds, typically organic compounds, that are depositable at process temperatures where a deposit can be characterized as a non-flowing liquid, a semi-solid or a solid. Primary tars are formed in the initial volatization process but are somewhat unstable and react chemically or dehydrogenate to form secondary and tertiary tars which are more difficult to react or re-hydrogenate than primary tars. In certain processes, the tars form solid particles of char and are no longer condensable but are still not desirable for commercial use.
A large effort has been made to reduce these tars by various means. Mechanical methods of tar removal include wet scrubbing and filtration. Wet scrubbing can be done at a range of temperatures with a variety of liquids in one or more stages and transfers the problem of tar condensation from the gas phase to a liquid phase. High temperature filtration is typically performed using metallic or ceramic candle filters and, while often effective for char particles, is generally ineffective for tar removal.
Thermal conversion of tars typically requires temperatures greater than 900° C.-1100° C. to achieve high conversion efficiencies. The energy required to attain this temperature is typically derived from oxidation of a portion of the process stream consuming some of the carbon and reducing conversion efficiency.
Another tar reduction method is catalytic conversion. Known catalysts are calcined dolomites and olivine, nickel-based catalysts, zirconium-based catalysts, and precious metal catalysts, with rhodium being the most promising.
In U.S. Pat. No. 4,865,625, Mudge disclosed the introduction of a gaseous oxidizing agent selected from the group consisting of air, oxygen, steam, and mixtures thereof in a catalytic reactor to eliminate tars. However, the addition of an oxidizing agent reduces the conversion rate to hydrocarbons (in this disclosure, conversion to hydrocarbons is the ratio of carbon as hydrocarbons-to-carbon as biomass.)
In U.S. Pat. No. 4,822,935, Scott disclosed a process to produce a methane-rich gas carried out at atmospheric pressure and low temperatures using direct catalytic hydrogasification. The Scott process disclosed a conversion of biomass to methane of 44% to 45% with co-generation of a minimum of 1.33% tar (in this disclosure, tar generation levels are by weight percent of moisture and ash free biomass). In a subsequent published test with different catalysts, with a non-methane hydrocarbon conversion of 22% to 26%, Scott generated between 4% to 7% levels of tar (Radlein, Mason, Piskorz, Scott, “Hydrocarbons from the Catalytic Pyrolysis of Biomass”, Energy and Fuels, 1991, 5). In both cases the tar levels are too high for commercial use.
Known methods of converting biomass to hydrocarbons suffer from low conversion efficiency due to tar mitigation solutions. Therefore a need still exists for a high efficiency process for biomass conversion to hydrocarbons.